This invention relates to the use of induction motors as generators and describes various means to make such use both practical and economical in applications such as hybrid electric vehicles and independent power generation.
Electric vehicles have suffered from disadvantages of limited driving range and the time required to recharge batteries. The recent introduction of hybrid electric vehicles by major manufacturers has demonstrated that when an electric drivetrain is combined with an engine powered generator, there are significant benefits in terms of range, emissions levels and fuel economy and there is no recharging time since the batteries are recharged from the on board generator while the vehicle is being driven. These range and emissions benefits are made possible by the manner in which the engines are used.
Engines used in many conventional applications such as transportation, construction and farming machinery, or engines used to power generators, must be sized so as to meet peak power requirements, but are generally run at average power levels which are far less than this peak power. In addition, the operating level typically varies widely in power and speed. If a smaller engine is run at the average power level of such a larger engine and at a nearly constant power and speed level, it will produce the same amount of energy over time as the larger engine, but it can have considerably better fuel economy and lower emissions than the larger engine. To be workable, such a configuration requires an energy reservoir to buffer the difference between the power required and the power supplied. By using such a smaller engine and coupling it to a DC generator which in turn charges a battery bank, this concept becomes a practical reality. A properly sized battery bank can be charged by the engine-generator at a fairly constant rate, while having the capability of providing peak power levels of, for example, over ten times the average or rated power of the system. For hybrid electric vehicles or other applications requiring mechanical power, a DC motor with motor controller can be connected to the battery bank. For independent AC power generation, a DC to AC inverter can be connected to the battery bank. This technology is referred to generally as “hybrid electric technology”, as it uses a combination of a prime mover—generator and an energy storage device. Generators that have traditionally been used in such hybrid electric systems are generally of the following types:                Synchronous—Automotive type (alternators), or commercial/industrial;        Shunt wound DC; and        Permanent magnet.Induction Motors and Generators—Overview        
It is well known that an induction motor can be used as a generator. Practical limitations, however, have restricted the use of such induction generators. The advantages of using induction motors as generators are many, and typically include the following:                1. Induction motors are readily available and very inexpensive per HP.        2. Induction motors are quite efficient (almost 90% even in small sizes for three phase units, and over 95% for larger units), and when run as generators, this efficiency is comparable. (Nailen, Richard L. How Induction Generators Work; Electrical Apparatus Magazine, June 1980, p. 55, 56.)        3. Induction motors are available in a wide power range, so this approach is very scalable.        4. Induction motors are very rugged, low maintenance machines. One reason is that they are brushless. There is no commutator or slip rings that must conduct current through brushes that are subject to wear and require maintenance. There are also no windings on the rotor that would be subject to mechanical stresses and failure.        
An AC motor or generator, whether synchronous or induction (asynchronous) operates on the principle of a rotating magnetic field. The rate of rotation of this field is equal to the synchronous speed of the machine, and this speed, Ns, is determined according to the formula Ns=120f/P where f equals the line frequency in Hz and P equals the number of poles in the machine. Therefore, for 60 Hz power input, the synchronous speed for a two pole machine is 3600 RPM, and 1800 RPM for a four pole machine. The rotor of a synchronous machine, whether motor or generator, operates at the synchronous speed. The rotor of an induction machine, if a motor, rotates below the synchronous speed and, if a generator, above the synchronous speed. (The difference between the synchronous speed and the rotor speed of an induction machine is called the slip.) This induction machine is simply an electric transformer whose magnetic circuit is separated by an air gap into two portions (rotor and stator) which move relative to one another. In the same manner that a transformer requires a magnetizing current (often referred to as an excitation current) in order to produce magnetic flux in the transformer core, an induction generator also requires a magnetizing current in order to produce flux in the rotor and stator cores, and therefore voltage at the output terminals. When connected to an existing AC power source, such as the utility power grid, this magnetizing current can be drawn from that source. However, if the generator is used to generate power independently, and is not connected to another AC power source, then another means must be used to provide this magnetizing current. This can be achieved by connecting capacitors to the output leads. (Bassett, E. D. and Potter, F. M. Capacitive Excitation for Induction Generators; Trans. Amer. Inst. Elec. Engrs., May 1935, Vol. 54, p. 540.) However, an induction generator can only be used to power resistive loads. (Nailen, Richard L. How Induction Generators Work; Electrical Apparatus Magazine, June 1980, p. 53.) If a reactive (inductive or capacitive) load is connected to an independent induction generator, the magnetizing current will be affected, and the output voltage will be lowered or raised, depending on the type and magnitude of the reactive load. The resulting instability of the output voltage renders the generator unsuitable for normal operation, since a fairly constant voltage is normally required. Various compensating schemes to offset the effect of reactive loads on the magnetizing current can become expensive and complex. Since reactive AC power loads are extremely common, the induction generator has been of limited usefulness for independent AC power generation.
This invention uses induction motors as generators in a manner which overcomes these limitations. Other features and advantages will appear from the following description in which the invention has been discussed in detail in conjunction with the accompanying drawings.